Letter press printing is a commonly used process wherein the printing surface of the printing plate stands out in relief and the non-printing parts are positioned below the printing surface. For high production letter press printing, rotary presses are employed in which each printing cylinder may carry several identical plates. As employed here, the term plate may also refer to an assembly of plates, or to an assembly of type and plates, as required to compose a single page. During each revolution, the cylinder will print as many impressions, (e.g. pages), on one side of the sheet which passes in contact with the cylinder, as the number of plates carried by it. Where more than one color is to be printed, separate cylinders each carrying a plate corresponding to a particular color are required. The cylinders must be accurately aligned with each other so that the plates will be in proper registration with each other. If multiple plate printing is carried out, the separate plates on successive cylinders that correspond to the different colors of a single impression must be in registration to assure that the different colors will appear in their appropriate precise location in the printed impression.
In the past and to some extent in current practice, the components that are to be assembled into a plate for printing one color of a single page, are locked in a metal frame that holds all the elements of the page securely in place. Papier mache, or the like, is placed over the assembled parts of the page and is squeezed down by hydraulic or mechanical pressure to form a stereotype matrix of the page. From the latter a stereotype plate is cast, usually in semicylindrical form, for mounting on the press cylinder of a rotary press.
More recently, a new technique has been developed wherein the stereotype plate is made of molded rubber or plastic and is disposed on a flexible mounting that can be fastened around the printing cylinder of a rotary press. Stereotypes made by this process are commonly referred to as flexographic plates or films and they can be made by a photographic process using a film that is sensitive to light. For example, the film may consist of a photopolymeric material which, when exposed to light shined through the negative of the image it is desired to transfer to the film, hardens where exposed. The non-exposed portions are subsequently removed by a chemical process, or otherwise, to leave the desired image in relief on the film.
Where multiple printing of the same image during each rotation of the print cylinder is required, as may be the case where labels are mass produced, the desired image must be provided on separate, precisely located portions of the flexographic film. This is done by accurately positioning the film at a succession of stations, using a step-and-repeat registration system in order to bring the different portions of the film opposite the original image-bearing sheet. The latter may be a negative or a diapositive, as the case may be. An image can then be formed on the film either through the use of imaging optics or by contact printing. The former is relatively more expensive than contact printing and often does not provide a sharply defined image.
When contact printing is carried out, in order to avoid damage to the film or to the original image-bearing sheet, film positioning must occur only when the two sheets are out of contact with each other. In order to provide an accurate image transfer, i.e. an accurately positioned exposure, the film is brought proximate the original image-bearing sheet at each station prior to being exposed to light. After the exposure is completed, the film is again spaced from the image-bearing sheet and thus transported to the next station.
Although various registration systems are in use today for carrying out the positioning process outlined above for contact-printing, many of them are considered to be cumbersome in use and difficult to service. As a result, the process of making a multiple-image, flexographic plate or film has heretofore tended to consume an undue amount of time. Further, presently available equipment is frequently unable to provide, reliably and repeatably, the accuracy required for proper color registration, i.e. a registration accuracy of the order of 3 mils or less. To the extent that such registration accuracy has been achieved, prior art apparatus capable of performing within these limits is costly, thereby offsetting the advantages of economy inherent in mass production flexographic printing.
A further failing of prior art registration systems resides in the lack of flexibility of such systems for operation in different modes. Clearly, whatever mode best suits the particular job requirements, e.g. manual, semi-automatic, or fully automatic operation will provide the best results at the lowest cost. However, prior art registration systems are usually dedicated to only one of these operating modes and hence unable to take advantage of different types of situations.